Essential oil (essential oil or natural essential oil) is an organic substance comprising a volatile odorant substance, extracted form a plant. The essential oil is obtained by treating a plant as a raw material with steam distillation, compression, extraction and the like, and recovering an oil comprising volatile odorant ingredients.
Of essential oils, citrus oil using citrus as a raw material is that monoterpene hydrocarbon which is a scent ingredient contained in citrus, and other terpene hydrocarbons are unstable to heat and light. For this reason, a so-called “cold press method” which recovers an oil by compressing citrus (particularly, coat of citrus) without applying heat is generally widely employed as a preparation method of the citrus oil.
The essential oil obtained above is generally used without modification as flavor or fragrance in various uses including cosmetic or food. However, in the case where troubles such as coloration, precipitation, turbidity, deteriorated odor and the like occurred by chemical change and alternation of ingredients contained in the essential oil during storage, the essential oil is purified and used. As the purification method in such a case, a method by distillation, a method of treating with a porous adsorbent, a cleaning treatment method, a filtration treatment method, and the like are employed. In the case of treating with a porous adsorbent, inorganic adsorbents such as active carbon, silica gel, alumina and zeolite, organic adsorbents such as styrene-divinylbenzene copolymer adsorbent and acryl absorbent, ion-exchange resins, porous glasses, and the like are used (see Non-Patent Document 1).
The essential oil is derived from natural plant, and should be inherently safe. However, a harmful contaminant such as agricultural chemicals, different from altered substances formed by chemical change and the like of the ingredients during storage and impurities originally contained in the essential oil, are sometimes detected, and the countermeasure becomes the problem.
The cause that agricultural chemicals are detected from the essential oil includes the case where agricultural chemicals used in the course of growth of a plant or used in the course of storage and distribution of the plant harvested are attached to and adsorbed on the plant as a raw material, and the agricultural chemicals move into the essential oil and remain therein, and the case where a plant absorbs agricultural chemicals not decomposed and accumulated in the soil among agricultural chemicals used in the past, from root, and the agricultural chemicals move into the essential oil and remain therein.
A method of conducting supercritical extraction of dried plant raw material, a method of conducting extraction treatment of a plant using a hydrophobic extractant, and the like are known as a method for producing a plant extract in which agricultural chemicals do not remain. However, those methods are a method that residual agricultural chemicals are avoided from being contained in a plant extract when preparing a plant extract by an extraction treatment of a plant. Therefore, the method cannot be applied to an essential oil, particularly citrus oil, in which scent ingredients are recovered by a compression method without using extraction.
A method wherein a plant extract, in which organic chlorine compound type agricultural chemicals remain therein, is dissolved in a mixed solution of a lower aliphatic alcohol and water in a volume ratio of from 10:90 to 80:20, and then the resulting solution is contacted with a porous adsorbing resin having a modal radius of pores of from 30 to 120 angstroms, the porous adsorbing resin comprising a three-dimensional copolymer of styrene and divinylbenzene and the like, thereby adsorbing residual agricultural chemicals in the solution on the adsorbing resin, and the plant extract is recovered from the treated solution is known as a method for removing agricultural chemicals remained in a plant extract (Patent Document 1). However, in the case of this method, effective ingredients having similar properties to those of residual agricultural chemicals, contained in the plant extract are adsorbed on the absorbing resin together with the residual agricultural chemicals. As a result, there is a problem that absorption loss is large, resulting in decrease in yield of a plant extract.
Furthermore, this method is a method for removing residual agricultural chemicals from a plant extract obtained by previously conducting an extraction treatment, such as a tea extract, and an essential oil obtained by compression without using extraction is not an intended material to be treated.
A method of cleaning an essential oil with an alkaline aqueous solution (Patent Document 2) and a method of treating an essential oil with a strong ion-exchange resin (Patent Document 3) are known as the method for removing residual agricultural chemicals from an essential oil.
However, the method of cleaning an essential oil with an alkaline aqueous solution has the problem that alkali-soluble flavor ingredients in the essential oil move into the alkaline aqueous solution, causing poor balance of flavor of the essential oil and considerable decrease in flavor strength, and the quality of the essential oil is greatly decreased.
The method of treating an essential oil with a strong ion-exchange resin has various problems that (a) because the strong ion-exchange resin is hydrophilic while the essential oil is hydrophobic, it is difficult to increase contact efficiency of the essential oil to the strong ion-exchange resin, and removal efficiency of residual agricultural chemicals is low; (b) there is a concern that water incorporates into the essential oil; (c) there is a concern that aldehydes which are important flavor ingredients in the essential oil are ethanolated, thereby inducing a polymerization reaction; (d) in the case where the strong ion-exchange resin is a cationic ion-exchange resin, loss of acidic flavor compounds in the essential oil occurs, on the other hand, in the case where the strong ion-exchange resin is an anionic ion-exchange resin, loss of alkaline flavor compounds in the essential oil occurs, and poor balance of flavor of the essential oil and decrease in flavor strength are easy to occur; and (e) there is a concern that dissolution of the strong ion-exchange resin occurs by the essential oil.
As described above, Non-Patent Document 1 describes that in the case where coloration, precipitation, turbidity, deteriorated odor and the like occurred by chemical change and alternation of ingredients contained in the essential oil during storage, the essential oil is purified with an adsorbent such as active carbon, thereby improving flavor. However, Non-Patent Document 1 does not describe the preparation of an essential oil having excellent safety, free of contaminant such as agricultural chemicals by treating an essential oil contaminated with a contaminant such as agricultural chemicals with active carbon.
Under the above circumstances, development of a method capable of producing a purified essential oil having excellent safety and high quality with good productivity by smoothly removing a contaminant such as agricultural chemicals contained in the essential oil in a simple operation while maintaining good balance of flavor which an essential oil originally has and flavor strength without causing loss and deterioration of flavor ingredients contained in the essential oil is being demanded.